June 15, 2020
For educational institutions
Atomic Structure timeline
⟶ Updated 25 Nov 2017 ⟶
List of edits
John Dalton: Atomic Theory He proposed the atomic theory and stated that all matter is composed of small particles called atom; atoms of a given element has an identical size, mass and other properties; atoms cannot be cannot be destroyed, subdivided or created; in chemical reactions, atoms are separated, combined or rearranged; and atoms of different elements can combine in whole-number ratio to form a chemical compound.
Dmitri Mendeleev: Periodic Table He organized the initial 63 elements into the periodic table by the atomic mass. The format of the rows and columns are still used today.
G.J. Stoney: The electron He calculated the magnitude of an atom and proposed electricity was created by negative particles called “electron”. This was supported by the collection of data from the kinetic theory of gas and electrolysis of water.
Marie Curie: Radioactivity and new elements She conducted experiments with uranium and thorium to research about radioactivity and its process of decay and also discovered two new elements - polonium and radium. She is the first person to win 2 Nobel Prizes and the first female to win the prize.
Ernest Rutherford: Radioactivity Through the development of different innovative techniques, He discovered two different radioactive rays -alpha and beta. He also later discovered the radioactive gas - radon.
Ernest Rutherford: The Rutherford model He proposed a model of the atom which suggests that the atom contained a positively charged nucleus which was surrounded by small negatively charged elections. He came to this conclusion from directing the famous Geiger- Marsden experiment which is also known as the gold leaf experiment. Alpha rays were aimed at gold leaf but instead of passing through it, it was repelled or emerged at different angles.
H.G.J Moseley: Atomic Structure He determined that "The atomic number of an element is equal to the number of protons in the nucleus". He also came to the conclusion that there were 3 unknown elements (there are actually 4 unknown ) between aluminium and gold.
Francis William Aston: Isotopes With a mass spectrometer, he discovered the existence of isotopes and created the “whole number rule” which states “the masses of the isotopes are whole number multiples of the mass of the hydrogen atom”
Niels Bohr: Atomic Structure He proposed a theory suggesting that how electrons are able to stably orbited the nucleus and stating that electrons have to move in a fixed energy and size. This theory is a modified version of Rutherford’s model.
James Chadwick: The Neutron Through experimenting with Beryllium by projecting alpha particles towards it and “filtering” it through paraffin wax, he discovered a particle that did not consist of a charge, which he named “neutron”
The Standard Model: The current model of the atom states that an atom consists of electrons orbiting the nucleus which exist in the electron shell. The electron is negatively charged. The nucleus consists of protons and neutron and the proton is positively charged whilst the neutron has no charge.
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